1. Field of the Invention
The invention relates generally to a differential global positioning system (DGPS) and more particularly to a DGPS system using GPS almanac data for determining the locations-in-space of GPS signal sources and the DGPS corrections for providing a differentially corrected location.
2. Description of the Prior Art
The global positioning system (GPS) is a satellite based location and time transfer system developed by the United States government and available free of charge to all users. A GPS user location is based upon one-way ranging between the user and GPS satellites. The GPS satellites transmit signals having the times-of-transmission and orbital parameters for their respective time variable locations-in-space. A GPS receiver measures the ranges to typically four satellites simultaneously in-view by correlating the incoming GPS signals to internal GPS replica signals and measuring the received phases against an internal clock. These ranges are generally called pseudoranges because they include a term for the error of the internal clock. The pseudoranges are then used in location equations having calculated quantities for the locations-in-space for several satellites and the directional cosines from the user location to the satellites. With four equations for four GPS satellites, respectively, the GPS receiver can resolve the four unknowns of a three dimensional geographical user location and a correction to the internal clock. Fewer than four satellites are required if other location information is available. However, many GPS receivers today use up to twelve GPS satellites in an overdetermined solution in order to improve location accurary.
Two types of orbital parameters are transmitted for determining locations-in-space for the satellites, almanac data and ephemeris data. The almanac data includes relatively few parameters and is generally sufficient for determining locations-in-space to a few kilometers. Each GPS satellite broadcasts the almanac data for all the GPS satellites on a twelve and one-half minute cycle. Almanac data is updated every few days and is useful for several weeks. Because of its relatively long lifetime, the almanac data is typically used by GPS receivers that have been off for more than a few hours for determining which GPS satellites are in-view. However, as the inaccuracy of the location-in-space of a GPS satellite transfers to an inaccuracy in the user location, almanac data is not used by existing GPS receivers for ranging. The ephemeris data provides relatively more parameters and is much more accurate. Typically, current ephemeris data is sufficient for determining locations-in-space to a few meters or a few tens of meters at current levels of selective availability. Each GPS satellite broadcasts its own ephemeris data on a thirty second cycle. Ephemeris data is updated each hour. However, after about two hours the accuracy of the ephemeris data begins degrading. Typically, ephemeris data that is more than about two to four hours old is not used for ranging.
A stand alone accuracy of existing commercial GPS receivers is a typically within about twenty meters or within about one-hundred meters with selective availability (SA). In order to achieve these accuracies, existing GPS receivers use the current ephemeris data for determining the locations-in-space of the GPS satellites in location equations. In order to improve the stand alone accuracy with or without SA, differential GPS (DGPS) systems use a GPS reference station having an accurately known reference location for providing DGPS corrections. The DGPS corrections are computed from the differences between the ranges that the GPS reference station measures in a conventional manner and ranges that are calculated based upon the known location. A remote GPS user receiver receives the DGPS corrections with radio signals for correcting the raw pseudoranges that it measures to the same GPS satellites at the same times as the GPS reference station. Using such DGPS corrections, GPS receivers can obtain a location accuracy within a meter or even better. Alternatively, the raw pseudoranges can be transmitted or put onto a disk and carried to another site for differential correction.
Several applications for GPS receivers require or make desirable a fast time to a first location fix after having been off or in a standby mode for more than a few hours. One of the problems in getting a fast location fix is that the GPS receiver needs to collect new ephemeris data before the location can be computed. Typically, the ephemeris data is obtained directly from the GPS satellites in the GPS signals. However, up to about thirty seconds is required to acquire ephemeris data in this manner. It has been proposed that this thirty seconds can be eliminated in one of two ways. First, if location is not needed at the remote, the solution can be computed at a network base station where current ephemeris data is available. In this case, the raw pseudoranges are sent to the base station along with the satellite identifications and times. Second, if the location is needed at the remote, the base station sends the current ephemeris data including the satellite identifications and times to the remote for location determination. These schemes are attractive for real time DGPS systems where radio equipment is already required. Unfortunately, these proposals require the transmission and reception of a relatively long data string for the ephemeris data of approximately 1500 bits per satellite or 15000 bits for ten in-view satellites. Further, the transmission and reception must be accomplished every few hours in order that the ephemeris data be up-to-date.
Hatch et al. in U.S. Pat. No. 5,764,184 discloses a method and system for post-processing DGPS system satellite positional data. Hatch recognizes that almanac data is sufficiently accurate for computation of directional cosines because the high altitude of the GPS satellites renders the directional cosines relatively insensitive to errors in satellite locations-in-space. The almanac-based directional cosines are then used in post-processing to map reference station corrections to user receiver stand-alone position information for correcting the user position. An advantage asserted by Hatch is that substantially less information is required to be saved by the user for post-processing. However, Hatch does not address the issue of the acquisition time for the user receiver for receiving ephemeris data for ranging in order to obtain the stand-alone position.
There is a need for a differential GPS system where a remote GPS user receiver has a fast time to first fix without a requirement of receiving ephemeris data.